1. Field of the Invention
The present invention relates to a fixing device configured to be incorporated in an image forming apparatus and the image forming apparatus.
2. Description of the Related Art
There have been known fixing devices of electromagnetic induction heating type used in an image forming apparatus such as copiers, printers, and facsimile machines. For example, Japanese Patent Application Laid-open No. 2007-328159 discloses an electromagnetic induction heating fixing device. The fixing device of this type is mainly includes a fixing member such as a fixing roller and a fixing belt, a pressing member that is in pressure contact with the fixing member to form a nip portion, and an induction heating unit that faces the outer circumferential surface of the fixing member to heat the fixing member by electromagnetic induction. The induction heating unit includes an excitation coil, a core that covers the excitation coil, a coil guide that holds the excitation coil and faces the fixing member, and the like.
Energizing the excitation coil of the induction heating unit causes a magnetic flux to be formed around the heat generating layer of the fixing member or the heat generating layer of a heating member abutting against the fixing member. The heat generating layer is thus heated by electromagnetic induction, resulting in the fixing member being directly or indirectly heated. Accordingly, toner on a recording medium in contact with the fixing member at the fixing nip portion is heated and melted, and thereby fixed on the recording medium.
The heat generating layer is required to be thin because it also forms the fixing nip portion. Thus, the heat generating layer as well as the sleeve layer is susceptible to damage when the heat generating layer is defective due to scratches on the material or malfunctions such as runaway due to overheating. Damage to the sleeve layer causes a broken piece of the thin metal layer such as the heat generating layer, raising the possibility of failure of the apparatus resulting from dropping broken pieces or injury of the user touching those broken pieces.
On the other hand, the heat insulating elastic layer, which is located closer to the inner circumferential surface than the heat generating layer is, rotates while being pushed at a high pressure by the pressing member to form a nip. Accordingly, the heat insulating elastic layer is susceptible to wear, and may be damaged when used beyond the expected service life. Damage to the heat insulating elastic layer also raises the possibility of failure of the apparatus due to dropping broken pieces. Further, along with the damage to the heat insulating elastic layer, damage is likely to be caused to the surface layer or the sleeve layer located closer to the outer circumferential surface than the heat insulating elastic layer is.